1. Field of the Invention
The present invention relates to, after a conversion reaction process for producing trichlorosilane by reacting hydrogen with silicon tetrachloride discharged from a polycrystalline silicon producing process, a method for separating and recovering conversion reaction gas to separate and recover trichlorosilane and silicon tetrachloride from the discharged gas and then to recover disilicon hexachloride or the like.
Priority is claimed on Japanese Patent Application No. 2007-309859, filed Nov. 30, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
Disilicon hexachloride is useful as a raw material of an amorphous silicon film, a raw material of optical glass fiber, or a raw material of disilanes. As a method for producing disilicon hexachloride, there is known a method for separating and recovering disilicon hexachloride by cooling and condensing a mixed gas of polychlorosilane obtained by chlorination of a powdered alloy containing silicon, and separating by distillation (Japanese Patent Application Laid-Open No. 59-195519). In addition, there is known a method for producing disilicon hexachloride by reacting ferrosilicon with chlorine using a mixing horizontal reaction tube (Japanese Patent Application Laid-Open No. 60-145908).
In all the production methods, low purity silicon (metal silicon grade: purity 98 wt %) is used as a raw material, and it is difficult to avoid impurity from the raw material. Accordingly, there is a problem in that it is difficult to obtain a high-purity product. Particularly, when aluminum is mixed therein, chloride (TiCl4, AlCl3) thereof has a boiling point close to that of disilicon hexachloride. Therefore, it is difficult to distill and separate the chloride, and thus it is difficult to obtain high-purity disilicon hexachloride.
As a production method without such problems, there is known a method for obtaining high-purity disilicon hexachloride that does not include titanium or aluminum substantially, by distilling a high molecular silicon chloride compound (polymer) that is a by-product produced in a polycrystalline silicon producing process and recovering disilicon hexachloride (International Patent Application No. WO002/012122).
In the method for recovering disilicon hexachloride from a polymer by-produced in the polycrystalline producing process, the composition of the polymer depends on production conditions (process temperature, amount of trichlorosilane/hydrogen gas introduced) of the polycrystal silicon production process. Accordingly, when the production conditions are not constant, there is a problem in that it is difficult to stably obtain disilicon hexachloride.
Specifically, a polycrystalline silicon production process is generally a batch process in which a reaction period of 3 days to 6 days is used, and production conditions according to a growth process are controlled. Accordingly, the composition of reacted discharged gas is not uniform. For this reason, a content of disilicon hexachloride contained in the polymer is not uniform, a distillation process is not stabilized, and it is difficult to stably produce disilicon hexachloride.